MedB-1, a human tumor cell line derived from a primary mediastinal large B-cell lymphoma

Specific interactions of BCL-2 family proteins mediate sensitivity to BH3-mimetics in diffuse large B-cell lymphoma

The BCL-2-specific inhibitor, ABT-199 (venetoclax) has exhibited remarkable clinical activity in nearly all cases of chronic lymphocytic leukemia. In contrast, responses are usually much less in diffuse large B-cell lymphoma (DLBCL), despite high level expression of BCL-2 in over 40% of cases, indicating that co-expression of related anti-apoptotic BCL-2 family proteins may limit the activity of ABT-199. We have investigated the roles of BCL-2 proteins in DLBCL cells using a panel of specific BCL-2 homology 3 (BH3)-mimetics and identified subgroups of these cells that exhibited marked and specific dependency on either BCL-2, BCL-X_L or MCL-1 for survival. Dependency was associated with selective sequestration of the pro-apoptotic proteins BIM, BAX and BAK by the specific anti-apoptotic BCL-2 protein which was important for cellular survival. Sensitivity to BH3-mimetics was independent of genetic alterations involving the BCL-2 family and only partially correlated with protein expression levels. Treatment with ABT-199 displaced BAX and BIM from BCL-2, subsequently leading to BAK activation and apoptosis. In contrast, apoptosis induced by inhibiting BCL-X_L with A1331852 was associated with a displacement of both BAX and BAK from BCL-X_L and occurred independently of BIM. Finally, the MCL-1 inhibitor S63845 induced mainly BAX-dependent apoptosis mediated by a displacement of BAK, BIM and NOXA from MCL-1. In conclusion, our study indicates that in DLBCL, the heterogeneous response to BH3-mimetics is mediated by selective interactions between BAX, BAK and anti-apoptotic BCL-2 proteins.

Recurrent mutations of the exportin 1 gene (XPO1) and their impact on selective inhibitor of nuclear export compounds sensitivity in primary mediastinal B-cell lymphoma

Primary mediastinal B-cell lymphoma (PMBL) is an entity of B-cell lymphoma distinct from the other molecular subtypes of diffuse large B-cell lymphoma (DLBCL). We investigated the prevalence, specificity, and clinical relevance of mutations of XPO1, which encodes a member of the karyopherin-β nuclear transporters, in a large cohort of PMBL. PMBL cases defined histologically or by gene expression profiling (GEP) were sequenced and the XPO1 mutational status was correlated to genetic and clinical characteristics. The XPO1 mutational status was also assessed in DLBCL, Hodgkin lymphoma (HL) and mediastinal gray-zone lymphoma (MGZL).The biological impact of the mutation on Selective Inhibitor of Nuclear Export (SINE) compounds (KPT-185/330) sensitivity was investigated in vitro. XPO1 mutations were present in 28/117 (24%) PMBL cases and in 5/19 (26%) HL cases but absent/rare in MGZL (0/20) or DLBCL (3/197). A higher prevalence (50%) of the recurrent codon 571 variant (p.E571K) was observed in GEP-defined PMBL and was associated with shorter PFS. Age, International Prognostic Index and bulky mass were similar in XPO1 mutant and wild-type cases. KPT-185 induced a dose-dependent decrease in cell proliferation and increased cell-death in PMBL cell lines harboring wild type or XPO1 E571K mutant alleles. Experiments in transfected U2OS cells further confirmed that the XPO1 E571K mutation does not have a drastic impact on KPT-330 binding. To conclude the XPO1 E571K mutation represents a genetic hallmark of the PMBL subtype and serves as a new relevant PMBL biomarker. SINE compounds appear active for both mutated and wild-type protein. Am. J. Hematol. 91:923-930, 2016. © 2016 Wiley Periodicals, Inc.

Genomic Landscape of Primary Mediastinal B-Cell Lymphoma Cell Lines

Primary mediastinal B-Cell lymphoma (PMBL) is a recently defined entity comprising ~2–10% non-Hodgkin lymphomas (NHL). Unlike most NHL subtypes, PMBL lacks recurrent gene rearrangements to serve as biomarkers or betray target genes. While druggable, late chemotherapeutic complications warrant the search for new targets and models. Well characterized tumor cell lines provide unlimited material to serve as preclinical resources for verifiable analyses directed at the discovery of new biomarkers and pathological targets using high throughput microarray technologies. The same cells may then be used to seek intelligent therapies directed at clinically validated targets. Four cell lines have emerged as potential PMBL models: FARAGE, KARPAS-1106P, MEDB-1 and U-2940. Transcriptionally, PMBL cell lines cluster near c(lassical)-HL and B-NHL examples showing they are related but separate entities. Here we document genomic alterations therein, by cytogenetics and high density oligonucleotide/SNP microarrays and parse their impact by integrated global expression profiling. PMBL cell lines were distinguished by moderate chromosome rearrangement levels undercutting cHL, while lacking oncogene translocations seen in B-NHL. In total 61 deletions were shared by two or more cell lines, together with 12 amplifications (≥4x) and 72 homozygous regions. Integrated genomic and transcriptional profiling showed deletions to be the most important class of chromosome rearrangement. Lesions were mapped to several loci associated with PMBL, e.g. 2p15 (REL/COMMD1), 9p24 (JAK2, CD274), 16p13 (SOCS1, LITAF, CIITA); plus new or tenuously associated loci: 2p16 (MSH6), 6q23 (TNFAIP3), 9p22 (CDKN2A/B), 20p12 (PTPN1). Discrete homozygous regions sometimes substituted focal deletions accompanied by gene silencing implying a role for epigenetic or mutational inactivation. Genomic amplifications increasing gene expression or gene-activating rearrangements were respectively rare or absent. Our findings highlight biallelic deletions as a major class of chromosomal lesion in PMBL cell lines, while endorsing the latter as preclinical models for hunting and testing new biomarkers and actionable targets.

CK1δ in lymphoma: gene expression and mutation analyses and validation of CK1δ kinase activity for therapeutic application

The prognosis of lymphoid neoplasms has improved considerably during the last decades. However, treatment response for some lymphoid neoplasms is still poor, indicating the need for new therapeutic approaches. One promising new strategy is the inhibition of kinases regulating key signal transduction pathways, which are of central importance in tumorigenesis. Kinases of the CK1 family may represent an attractive drug target since CK1 expression and/or activity are associated with the pathogenesis of malignant diseases. Over the last years efforts were taken to develop highly potent and selective CK1-specific inhibitor compounds and their therapeutic potential has now to be proved in pre-clinical trials. Therefore, we analyzed expression and mutational status of CK1δ in several cell lines representing established lymphoma entities, and also measured the mRNA expression level in primary lymphoma tissue as well as in non-neoplastic blood cells. For a selection of lymphoma cell lines we furthermore determined CK1δ kinase activity and demonstrated therapeutic potential of CK1-specific inhibitors as a putative therapeutic option in the treatment of lymphoid neoplasms.

Malignant hematopoietic cell lines: in vitro models for the study of primary mediastinal B-cell lymphomas

Primary mediastinal B-cell lymphoma (PMBL) is a highly aggressive disease with a unique set of biological, clinical, morphological, immunological and in particular genetic features that in the molecular era of defining lymphomas clearly distinguishes it as a separate entity from other diffuse large B-cell lymphomas (DLBCL). A precise molecular diagnosis of PMBL can be achieved by gene expression profiling. The signature gene expression profile of PMBL is more closely related to classic Hodgkin lymphoma (cHL) than to other DLBCL subgroups. A number of common genetic aberrations in PMBL and cHL further underscore their close relationship. To investigate the pathobiology of lymphomas in depth, many groups have turned to cell lines that are suitable models facilitating molecular studies and providing unique insights. For the purposes of the current perspective, we focus on four bona fide PMBL-derived cell lines (FARAGE, KARPAS-1106, MEDB-1, U-2940) that we identified and validated as such through hierarchical cluster analysis among a large collection of leukemia-lymphoma cell lines. These gene expression profiles showed that the four PMBL cell lines represent a distinct entity and are most similar to cHL cell lines, confirming derivation from a related cell type. A validated cell line resource for PMBL should assist those seeking druggable targets in this entity. This review aims to provide a comprehensive overview of the currently available cellular models for the study of PMBL.

Distinct isoform of FABP7 revealed by screening for retroelement-activated genes in diffuse large B-cell lymphoma

Remnants of ancient transposable elements (TEs) are abundant in mammalian genomes. These sequences harbor multiple regulatory motifs and hence are capable of influencing expression of host genes. In response to environmental changes, TEs are known to be released from epigenetic repression and to become transcriptionally active. Such activation could also lead to lineage-inappropriate activation of oncogenes, as one study described in Hodgkin lymphoma. However, little further evidence for this mechanism in other cancers has been reported. Here, we reanalyzed whole transcriptome data from a large cohort of patients with diffuse large B-cell lymphoma (DLBCL) compared with normal B-cell centroblasts to detect genes ectopically expressed through activation of TE promoters. We have identified 98 such TE-gene chimeric transcripts that were exclusively expressed in primary DLBCL cases and confirmed several in DLBCL-derived cell lines. We further characterized a TE-gene chimeric transcript involving a fatty acid-binding protein gene (LTR2-FABP7), normally expressed in brain, that was ectopically expressed in a subset of DLBCL patients through the use of an endogenous retroviral LTR promoter of the LTR2 family. The LTR2-FABP7 chimeric transcript encodes a novel chimeric isoform of the protein with characteristics distinct from native FABP7. In vitro studies reveal a dependency for DLBCL cell line proliferation and growth on LTR2-FABP7 chimeric protein expression. Taken together, these data demonstrate the significance of TEs as regulators of aberrant gene expression in cancer and suggest that LTR2-FABP7 may contribute to the pathogenesis of DLBCL in a subgroup of patients.

STAT6-mediated BCL6 repression in primary mediastinal B-cell lymphoma (PMBL)

Primary mediastinal B-cell lymphoma (PMBL) is characterized by aberrant activation of JAK/STAT-signaling resulting in constitutive presence of phosphorylated STAT6 (pSTAT6). In primary PMBL samples pSTAT6 is only expressed in a sub-population of lymphoma cells in a pattern that is reminiscent of that of the BCL6 oncogene. Double-fluorescence staining was carried out to determine the association between these two proteins in ten primary PMBL cases and three available PMBL cell line models. Surprisingly, only a minute fraction of double-positive nuclei was observed, while each sample contained considerable fractions of single-positive pSTAT6 and BCL6 nuclei. The intratumoral coexistence of BCL6+/pSTAT6− and BCL6−/pSTAT6+ subpopulations suggests a negative interaction between these factors. In silico screening of the STAT6 /BCL6 promoters for DNA consensus binding sites identified five STAT-binding-sites in the BCL6 promoter. We confirmed STAT6 binding to the BCL6 promoter in vitro and in vivo by band shift / super shift assays and chromatin immunoprecipitations. Using BCL6 luciferase reporter assays, depletion of STAT6 by siRNA, and ectopic overexpression of a constitutive active STAT6 mutant, we proved that pSTAT6 is sufficient to transcriptionally repress BCL6. Recently developed small molecule inhibitors 79-6 and TG101348 that increases BCL6 target gene expression and decreases pSTAT6 levels, respectively, demonstrate that a combined targeting results in additive efficacy regarding their negative effect on cell viability.

The delineated pSTAT6-mediated molecular repression mechanism links JAK/STAT to BCL6-signaling in PMBL and may carry therapeutic potential.

Recurrent somatic mutations of PTPN1 in primary mediastinal B cell lymphoma and Hodgkin lymphoma

Classical Hodgkin lymphoma and primary mediastinal B cell lymphoma (PMBCL) are related lymphomas sharing pathological, molecular and clinical characteristics. Here we discovered by whole-genome and whole-transcriptome sequencing recurrent somatic coding-sequence mutations in the PTPN1 gene. Mutations were found in 6 of 30 (20%) Hodgkin lymphoma cases, in 6 of 9 (67%) Hodgkin lymphoma-derived cell lines, in 17 of 77 (22%) PMBCL cases and in 1 of 3 (33%) PMBCL-derived cell lines, consisting of nonsense, missense and frameshift mutations. We demonstrate that PTPN1 mutations lead to reduced phosphatase activity and increased phosphorylation of JAK-STAT pathway members. Moreover, silencing of PTPN1 by RNA interference in Hodgkin lymphoma cell line KM-H2 resulted in hyperphosphorylation and overexpression of downstream oncogenic targets. Our data establish PTPN1 mutations as new drivers in lymphomagenesis.

The molecular pathogenesis of primary mediastinal large B-cell lymphoma

Primary mediastinal large B-cell lymphoma (PMBCL) is a recognized non-Hodgkin lymphoma entity with unique pathologic, clinical, and molecular characteristics distinct from those of other diffuse large B-cell lymphomas. Immunohistochemical characterization and molecular studies strongly suggest that PMBCL is of germinal center or postgerminal center origin. Pivotal gene expression profiling work defined major deregulated pathway activities that overlap with Hodgkin lymphoma and prompted a more detailed analysis of candidate genes. In particular, the nuclear factor-κB and the Janus Kinase-Signal Transducer and Activator of Transcription signaling pathways are targeted by multiple genomic hits, and constitutive activity of both pathways can be considered molecular hallmark alterations of PMBCL. Moreover, data are emerging giving unique insight into remodeling of the epigenome that affects transcriptional regulation of a multitude of genes. More recently, the tumor microenvironment of PMBCL has shifted into focus based on a number of gene perturbations altering expression of surface molecules that contribute to immune escape. These findings highlight the importance of immune privilege in the pathogenesis of PMBCL and suggest that disrupting crosstalk between the tumor cells and the microenvironment might be a rational new therapeutic target in conjunction with traditional treatment strategies.

Recurrent mutations of the STAT6 DNA binding domain in primary mediastinal B-cell lymphoma

Primary mediastinal B-cell lymphoma (PMBL) is a separate entity of aggressive B-cell lymphoma, characterized by a constitutive activation of janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway, also observed in Hodgkin lymphoma. Although many cancers exhibit constitutive JAK-STAT pathway activation, mutations of STAT genes have not been reported in neoplasms. Here, we show that MedB-1 PMBL-derived and L1236 Hodgkin-derived cell lines and 20 of 55 (36%) PMBL cases harbor heterozygous missense mutations in STAT6 DNA binding domain, whereas no mutation was found in 25 diffuse large B-cell lymphoma samples. In 3 cases, somatic origin was indicated by the absence of the mutations in the nontumoral tissue. The pattern of STAT6 mutations was different from the classical features of somatic hypermutations. The mutant STAT6 proteins showed a decreased DNA binding ability in transfected HEK cells, but no decrease in expression of STAT6 canonical target genes was observed in PMBL cases with a mutated STAT6 gene. Although the oncogenic properties of STAT6 mutant proteins remain to be determined, their recurrent selection in PMBL strongly argues for their involvement in the pathogenesis of this aggressive B-cell lymphoma.

High expression of several tyrosine kinases and activation of the PI3K/AKT pathway in mediastinal large B cell lymphoma reveals further similarities to Hodgkin lymphoma

Mediastinal large B-cell (MBL) and classical Hodgkin lymphoma (HL) have several pathogenic mechanisms in common. As we recently observed aberrant tyrosine kinase (TK) activities in HL, we now analysed also MBL for such activities. Indeed, MBL and HL were the only B-cell lymphomas where elevated cellular phospho-tyrosine contents were typical features. Three TKs, JAK2, RON and TIE1, not expressed in normal B cells, were each expressed in about 30% of MBL cases, and 75% of cases expressed at least one of the TKs. Among the intracellular pathways frequently triggered by TKs, the PI3K/AKT pathway was activated in about 40% of MBLs and essential for survival of MBL cell lines, whereas the RAF/mitogen-activated protein kinase pathway seemed to be inhibited. No activating mutations were detected in the three TKs in MBL cell lines and primary cases. RON and TIE1 were each also expressed in about 35% and JAK2 in about 53% of HL cases. JAK2 genomic gains are frequent in MBL and HL but we observed no strict correlation of JAK2 genomic status with JAK2 protein expression. In conclusion, aberrant TK activities are a further shared pathogenic mechanism of MBL and HL and may be interesting targets for therapeutic intervention.

Target Sequence Accessibility Limits Activation-Induced Cytidine Deaminase Activity in Primary Mediastinal B-Cell Lymphoma

Activation-induced cytidine deaminase (AID) initiates somatic hypermutation (SHM) and class switch recombination (CSR) in activated B lymphocytes and is potentially implicated in genomic instability of B-cell malignancies. For unknown reasons, B-cell neoplasms often lack SHM and CSR in spite of high AID expression. Here, we show that primary mediastinal B-cell lymphoma (PMBL), an immunoglobulin (Ig)-negative lymphoma that possesses hypermutated, class-switched Ig genes, expresses high levels of AID with an intact primary structure but does not do CSR in 14 of 16 cases analyzed. Absence of CSR coincided with low Ig germ-line transcription, whereas high level germ-line transcription was observed only in those two cases with active CSR. Interleukin-4/CD40L costimulation induced CSR and a marked up-regulation of germ-line transcription in the PMBL-derived cell line MedB-1. In the PMBL cell line Karpas 1106P, CSR was not inducible and germ-line transcription remained low on stimulation. However, Karpas 1106P, but not MedB-1, had ongoing SHM of the Ig gene and BCL6. These genes were transcribed in Karpas 1106P, whereas transcription was undetectable or low in MedB-1 cells. Thus, accessibility of the target sequences seems to be a major limiting factor for AID-dependent somatic gene diversification in PMBL.

Human IL4I1 is a secreted L-phenylalanine oxidase expressed by mature dendritic cells that inhibits T-lymphocyte proliferation

Interleukin-4-induced gene 1 (IL4I1) was first described as a B-cell IL4-inducible gene and is highly expressed in primary mediastinal B-cell lymphomas. We established stable HEK293 clones expressing human and mouse IL4I1 to examine their biochemical properties and function. Both proteins were secreted into the culture medium, and we observed the secretion of endogenous human IL4I1 (hIL4I1) protein in a mediastinal lymphoma B-cell line, MedB-1. We showed that IL4I1 has l-amino acid oxidase activity, optimal at physiological pH and primarily directed toward phenylalanine. Immunohistochemical analysis of secondary lymphoid organs showed staining of germinal center macrophages and inflammatory myeloid cells. In vitro, functional enzyme was highest in mature dendritic cells (DCs), suggesting a role in antigen-presenting cell/T-lymphocyte cross-talk. Indeed, hIL4I1 inhibited the proliferation of CD3-stimulated T lymphocytes with a similar effect on CD4(+) and CD8(+) T cells. In contrast, memory T cells were more strongly affected by hIL4I1 and its catabolite H(2)O(2) than naive T cells. hIL4I1 inhibitory effect was dependent on enzymatic activity and H(2)O(2) production and associated with a transient down-regulation of TCRzeta expression. Altogether these data suggest IL4I1 as a new immunomodulatory enzyme produced by DCs.

Das mediastinale (thymische) großzellige B-Zell-Lymphom

Mediastinal B-cell lymphoma is a locally highly aggressive tumour which was first described in the early 1980s. The incidence is about 2-3% of all non-Hodgkin's lymphomas. The conceptional evolution of this lymphoma entity was hampered by its low incidence and the broad spectrum of morphological variants present. However, since mediastinal B-cell lymphoma has distinct morphological, immunological, genetic, and clinical features, it has been listed in the revised European-American Classification of Lymphoid Neoplasms (REAL-Classification) since 1994 as a variant of diffuse large B-cell lymphoma. In the World Health Organization classification of malignant lymphomas, mediastinal B-cell lymphoma is now listed with an own disease code (ICD-9679/3).

Gains ofREL in primary mediastinal B-cell lymphoma coincide with nuclear accumulation of REL protein

Gains or amplifications of the REL locus are frequently seen in primary mediastinal B-cell lymphoma (PMBL). In classical Hodgkin's lymphoma, genomic overrepresentation of REL correlated with nuclear REL protein accumulation. To investigate the correlation between REL gene copies and its RNA and protein expression in PMBL, we analyzed genomic, transcriptional, and protein levels in 20 PMBLs and the PMBL derived cell lines MedB-1 and Karpas1106P. We found gains/amplifications in 75% of the PMBLs by fluorescence in situ hybridization (FISH) and genomic REL overrepresentation in the PMBL lines. Three of the five PMBLs with amplifications displayed elevated REL transcripts, while only 3/10 PMBLs with gains showed increased REL transcripts by real-time PCR. One PMBL without gains displayed increased REL transcription. REL protein expression exhibited a variable pattern across the PMBLs except for a single case that was completely negative by immunohistochemistry despite having gained REL. Although transcript levels were generally low and nuclear REL staining was weak in the lymphoma cell lines, these nevertheless exhibited high NF-kappaB activation. By fluorescence immunophenotyping and interphase cytogenetics as a tool for investigation of neoplasms, genomic gains/amplifications of REL significantly correlated with nuclear REL expression (P < 0.05). In conclusion, the frequent genomic overrepresentation of REL in PMBL does not necessarily trigger an increased transcription/translation of REL. However, combined genomic and protein analysis revealed a significant association of gained REL and nuclear REL accumulation at the single cell level.

U-2940, a human B-cell line derived from a diffuse large cell lymphoma sequential to Hodgkin lymphoma

Several patterns of association between Hodgkin and non-Hodgkin lymphomas are recognized, some of which support a common cellular origin or shared transformation events for both malignancies. We describe the U-2940 cell line derived from a diffuse large B-cell lymphoma with some features consistent with mediastinal large B-cell lymphoma, clinically apparent 1 month after the initial course of chemotherapy for Hodgkin's disease, fulfilling the criteria for composite malignancies. U-2940 cells display a mature B phenotype with hypermutated IgH rearrangement typical of germinal/postgerminal center origin. The cell line is negative for Epstein-Barr virus and no evidence of t(14;18) was found. U-2940 cells display multiple chromosomal rearrangements similar to recurrent aberrations described in both Hodgkin and non-Hodgkin lymphomas, also partially shared by U-2932 derived from a B-cell lymphoma sequential to Hodgkin's disease. The original large B-cell lymphoma and the U-2940 cell line bear microsatellite instability, an abnormality associated with particular subtypes of non-Hodgkin lymphomas and found in tissues involved by Hodgkin lymphoma. Therefore, U-2940 cells bear several features known to occur in Hodgkin and in non-Hodgkin lymphomas, leading to the assumption that this cell line may constitute a useful tool to address elective pathways of lymphomagenesis and eventually the Hodgkin and non-Hodgkin lymphoma association.

Biallelic deletion within 16p13.13 includingSOCS-1in Karpas1106P mediastinal B-cell lymphoma line is associated with delayed degradation of JAK2 protein

Activity of Janus kinase 2 (JAK2) in the JAK2/STAT5 signaling pathway is critically controlled by suppressor of cytokine signaling-1 (SOCS-1). We have previously shown that SOCS-1 is biallelically mutated in the primary mediastinal B-cell lymphoma (PMBL) cell line MedB-1, resulting in impaired JAK2 degradation and sustained phospho-JAK2 action. SOCS-1 is frequently mutated in PMBL tumor primaries. Here, we report that the PMBL cell line Karpas1106P has a biallelic deletion of the SOCS-1 region on chromosome 16p13.13. By fluorescence in situ hybridization and microsatellite analysis, this deletion was narrowed down to a range of 650 kb to 1.48 Mb. Like MedB-1, Karpas1106P harbors gains of the JAK2 gene on chromosomal region 9p24 and elevated levels of JAK2 mRNA. Nevertheless, JAK2 protein was not increased but constitutively phosphorylated in Karpas1106P cells. In analogy to MedB-1 cells, Karpas1106P cells exhibited a retarded degradation of de novo synthesized JAK2 protein revealed by pulse/chase experiments. Therefore, we conclude that loss of SOCS-1 function either by mutation or by the complete deletion of the gene plays an important role in the dysregulation of JAK/STAT signaling in Karpas1106P and PMBL.

Downregulation of internal enhancer activity contributes to abnormally low immunoglobulin expression in the MedB-1 mediastinal B-cell lymphoma cell line

Primary mediastinal B-cell lymphoma (PMBL) is a highly aggressive tumour with a unique pattern of clinical, morphological, immunological and genetic features distinct from other diffuse large B-cell lymphomas. PMBLs are characterized by a mature B-cell phenotype, but they typically lack immunoglobulin (Ig) gene expression. The PMBL cell line MedB-1 shares many characteristic properties of the primary tumour, including low-level Ig production despite a functionally rearranged IgVH gene and absence of 'crippling' mutations. In this study, a search was undertaken for reasons for downregulated Ig expression. Similar levels of the B-cell-specific transcription factors BOB.1/OBF.1 and PU.1 were found in MedB-1 cells to those in the Ig-producing UM-1 lymphoblastoid cell line. However, MedB-1 lacked the Oct2 transcription factor. Reporter assays showed that Ig-type promoters were active in MedB-1 cells. In contrast, activity of the intronic heavy chain enhancer was dramatically reduced. Ectopic expression of Oct2 was able partially to restore enhancer activity but transcription from the endogenous IgVH gene could not be rescued. Therefore, the role of epigenetic factors in the downregulation of Ig was investigated. Methylated histone 3 lysine 9, a reliable marker of chromatin silencing, was not detected in MedB-1 promoter and enhancer regions. Inhibition of DNA methyltransferase and of histone deacetylases also did not reactivate Ig production. These data suggest the existence of alternative mechanisms of Ig inhibition in MedB-1 cells, different from chromatin silencing and the lack of Oct2.

Biallelic mutation of SOCS-1 impairs JAK2 degradation and sustains phospho-JAK2 action in the MedB-1 mediastinal lymphoma line

Primary mediastinal B-cell lymphoma (PMBL) is a well-defined subtype of diffuse large B-cell lymphoma. Molecular cytogenetics revealed frequent gains of 9p24. JAK2, mapping in this region, is presently regarded as a candidate oncogene because expression profiling showed high Janus kinase-2 (JAK2) transcript levels and JAK2 was found to be constitutively phosphorylated in mediastinal B-cell lymphomas. We confirm that in the MedB-1 mediastinal B-cell line, harboring a trisomy 9, JAK2 transcription is elevated and the product is highly phosphorylated. However, JAK2 is not overexpressed at the protein level. On top, JAK2 protein turnover is even delayed. This unexpected finding coincides with a biallelic mutation of the suppressor of cytokine signaling-1 (SOCS-1) gene in this cell, which abrogates SOCS box function of the protein. Ectopic expression of wild-type (wt) SOCS-1 in MedB-1 leads to growth arrest and dramatic reduction of phospho-JAK2 and its downstream partner phospho-signal transducer and activator of transcription-5 (phospho-STAT5). Ultimately, the target gene cyclin D1 is repressed in transfectants while RB1, which is silenced in MedB-1, is induced. We conclude that, in MedB-1, action of phospho-JAK2 is sustained due to defective SOCS-1. Hence, SOCS-1 qualifies as a novel tumor suppressor. Of note, SOCS-1 mutations are also present in the parental tumor of MedB-1 and were detected in 9 of 20 PMBLs.

Constitutive STAT6 activation in primary mediastinal large B-cell lymphoma

Primary mediastinal large B-cell lymphoma (PMBL), currently recognized as a diffuse large B-cell lymphoma (DLBCL) subtype, shows increased expression of interleukin 4 (IL-4)/IL-13 signaling effectors and targets, suggesting constitutive activation of these pathways. We therefore investigated the functional state of the signal transducer and activator of transcription 6 (STAT6), mediating IL-4/IL-13 transcriptional effects. Constitutive STAT6 phosphorylation and DNA-binding activity were detected in PMBL cell lines but not DLBCL cell lines. Moreover, immunohistochemical analysis revealed nuclear phosphorylated STAT6 (P-STAT6) in 8 of 11 PMBL, compared with 1 of 10 DLBCL primary tumors (P =.01). IL-4 and IL-13 transcripts were absent in PMBL cell lines and expressed at low levels in tumors, indicating that, contrary to classical Hodgkin lymphoma (cHL), STAT6 activation is not due to an autocrine IL-4/IL-13 secretion. We demonstrated an amplification of the JAK2 gene in 2 of 6 PMBL cases, and showed higher JAK2 mRNA levels in PMBL compared with DLBCL (P =.005). The Janus kinase 2 (JAK2) was constitutively phosphorylated in the PMBL MedB1 cell line. MedB1 treatment with JAK2 inhibitor AG490 partially decreased STAT6 phosphorylation, suggesting that JAK2 is partially involved in STAT6 activation in these cells. Our findings highlight phosphorylated STAT6 as a characteristic distinguishing PMBL from DLBCL, but a common feature to PMBL and cHL, supporting the hypothesis of common pathogenic events in these 2 lymphomas.

Interleukin 4-induced gene 1 is activated in primary mediastinal large B-cell lymphoma

The molecular markers that distinguish primary mediastinal large B-cell lymphoma (PMBL) from nonmediastinal diffuse large B-cell lymphomas (NM-DLBLs) remain to be identified. Using cDNA representational difference analysis to compare PMBL and NM-DLBL transcripts, we isolated a cDNA fragment homologous to the mouse B-cell interleukin 4 (IL-4)-inducible gene FIG1 (interleukin 4-induced gene 1) transcript. The human FIG1 mRNA encodes a 567 amino acid protein that comprises a signal peptide and a large flavin-binding amino oxidase domain, and shares significant homology with secreted apoptosis-inducing L-amino acid oxidases. Northern blot studies showed that FIG1 mRNA expression is mainly restricted to lymphoid tissues. It is expressed at low levels in thymus, spleen, tonsils, and reactive lymph nodes, and is highly up-regulated in IL-4+CD40-activated tonsillar B cells. Interestingly, in human B-cell lines, FIG1 mRNA expression appeared restricted to the PMBL-derived MedB-1 and Karpas 1106 cell lines. Using real-time reverse transcriptase-polymerase chain reaction (RT-PCR), we demonstrated that all but one PMBL (16/17) displayed high FIG1 mRNA levels, whereas most NM-DLBLs (12/18) and all low-grade B-cell lymphomas tested (8/8) exhibited low FIG1 mRNA levels. The difference between PMBLs and NM-DLBLs was statistically significant (Fisher test; P =.0003). Southern blot studies did not show rearrangement of the FIG1 gene. FIG1 gene expression might be due to a constitutive activation of a cytokine signaling pathway in PMBL.

Mediastinal (thymic) large B-cell lymphoma: where do we stand?

Mediastinal (thymic) B-cell lymphoma (MBL) is a locally highly aggressive tumour that was first definitively described in the early 1980s. The incidence of MBL is low, which made disease characterisation difficult initially. However, MBL has several peculiar clinical, morphological, immunological, and genetic features. Collectively, these characteristics distinguish it from other diffuse, large B-cell lymphomas. Consequently, MBL has become a defined subtype of diffuse large B-cell lymphoma with its own code (9679/3) in the International Classification of Diseases. New insights into the biological and clinical aspects of MBL have been gained from the study of large numbers of cases. Nevertheless, the histogenesis of the disease is not yet fully understood. We review the available data on MBL with special emphasis on its morphological, immunological, and genetic properties. Also discussed are recent data on molecular genetics, biology, and treatment.